1. Field of the Invention
This invention relates to an integrated process, which treats at the surface, fluids recovered from a subsurface formation containing heavy crude oil or natural bitumen to produce a synthetic crude oil and also to produce the energy and reactants used in the recovery process. The quality of the treated oil is improved to such an extent that it is a suitable feedstock for transportation fuels and gas oil.
2. Description of the Prior Art
Worldwide deposits of natural bitumens (also referred to as "tar sands") and heavy crude oils are estimated to total more than five times the amount of remaining recoverable reserves of conventional crude [References 1,5]. But these resources (herein collectively called "heavy hydrocarbons") frequently cannot be recovered economically with current technology, due principally to the high viscosities which they exhibit in the porous subsurface formations where they are deposited. Since the rate at which a fluid flows in a porous medium is inversely proportional to the fluid's viscosity, very viscous hydrocarbons lack the mobility required for economic production rates.
In addition to high viscosity, heavy hydrocarbons often exhibit other deleterious properties which cause their upgrading into marketable products to be a significant refining challenge. These properties are compared in Table 1 for an internationally-traded light crude, Arabian Light, and three heavy hydrocarbons.
The high levels of undesirable components found in the heavy hydrocarbons shown in Table 1, including sulfur, nitrogen, metals, and Conradson carbon residue, coupled with a very high bottoms yield, require costly refining processing to convert the heavy hydrocarbons into product streams suitable for the production of transportation fuels.
TABLE 1 ______________________________________ Properties of Heavy Hydrocarbons Compared to a Light Crude Light Crude Heavy Hydrocarbons Arabian Cold Properties Light Orinoco Lake San Miguel ______________________________________ Gravity, .degree.API 34.5 8.2 11.4 -2 to 0 Viscosity, cp @ 100.degree. F. 10.5 7,000 10,700 &gt;1,000,000 Sulfur, wt % 1.7 3.8 4.3 7.9 to 9.0 Nitrogen, wt % 0.09 0.64 0.45 0.36 to 0.40 Metals, wppm 25 559 265 109 Bottoms (975.degree. F.+), 15 59.5 51 71.5 vol % Conradson carbon 4 16 13.1 24.5 residue, wt % ______________________________________
Converting heavy crude oils and natural bitumens to upgraded liquid hydrocarbons while still in a subsurface formation would address the two principal shortcomings of these heavy hydrocarbon resources--the high viscosities which heavy hydrocarbons exhibit even at elevated temperatures and the deleterious properties which make it necessary to subject them to costly, extensive upgrading operations after they have been produced. However, the process conditions employed in refinery units to upgrade the quality of liquid hydrocarbons would be extremely difficult to achieve in the subsurface. The injection of catalysts would be exceptionally expensive, the high temperatures used would cause unwanted coking in the absence of precise control of hydrogen partial pressures and reaction residence time, and the hydrogen partial pressures required could cause random, unintentional fracturing of the formation with a potential loss of control over the process.
A process occasionally used in the recovery of heavy crude oil and natural bitumen which to some degree converts in the subsurface heavy hydrocarbons to lighter hydrocarbons is in situ combustion. In this process an oxidizing fluid, usually air, is injected into the hydrocarbon-bearing formation at a sufficient temperature to initiate combustion of the hydrocarbon. The heat generated by the combustion warms other portions of the heavy hydrocarbon and converts a part of it to lighter hydrocarbons via uncatalyzed thermal cracking, which may induce sufficient mobility in the hydrocarbon to allow practical rates of recovery.
While in situ combustion is a relatively inexpensive process, it has major drawbacks. The high temperatures in the presence of oxygen which are encountered when the process is applied cause coke formation and the production of olefins and oxygenated compounds such as phenols and ketones, which in turn cause major problems when the produced liquids are processed in refinery units. Commonly, the processing of products from thermal cracking is restricted to delayed or fluid coking because the hydrocarbon is degraded to a degree that precludes processing by other methods.
U.S. patents, discussed below, disclose various processes for conducting in situ conversion of heavy hydrocarbons without reliance on in situ combustion. The more promising processes teach the use of downhole apparatus to achieve conditions within hydrocarbon-bearing formations to sustain what we designate as "in situ hydrovisbreaking," conversion reactions within the formation which result in hydrocarbon upgrading similar to that achieved in refinery units through catalytic hydrogenation and hydrocracking.
However, as a stand-alone process, in situ hydrovisbreaking has several drawbacks:
Analytic studies, presented in examples to follow, show that only partial conversion of the heavy hydrocarbon is achieved in situ, with the result that the liquid hydrocarbons produced might not be used in conventional refinery operations without further processing. PA1 In addition to the liquid hydrocarbons of interest, significant quantities of fluids are produced which are deleterious. PA1 The in situ process requires vast quantities of steam and reducing gases, which are injected into the subsurface formation to create the conditions required to initiate and sustain the conversion reactions. These injectants must be supplied at minimum cost for the overall process to be economic. PA1 Justheim, U.S. Pat. No. 3,327,782 modulates (heats or cools) hydrogen at the surface. In order to initiate the desired objectives of "distilling and hydrogenation" of the in situ hydrocarbon, hydrogen is heated on the surface for injection into the hydrocarbon-bearing formation. PA1 Hujsak, U.S. Pat. No. 4,448,251 teaches that hydrogen is obtained from a variety of sources and includes the heavy oil fractions from thc produced oil which can be used as reformer fuel. Hujsak also includes and teaches the use of forward or reverse in situ combustion as a necessary step to effect the objectives of the process. Furthermore, heating of the injected gas or fluid is accomplished on the surface, an inefficient means of heating compared to using a downhole combustion unit because of heat losses incurred during transportation of the heated fluids to and down the borehole. PA1 Stine, U.S. Pat. No. 4,448,251 utilizes a unique process which incorporates two adjacent, non-communicating reservoirs in which the heat or thermal energy used to raise the formation temperature is obtained from the adjacent reservoir. Stine utilizes in situ combustion or other methods to initiate the oil recovery process. Once reaction is achieved, the desired source of heat is from the adjacent zone. PA1 Gregoli, U.S. Pat. No. 4,501,445 teaches that a crude formation is subjected to fracturing to form "an underground space suitable as a pressure reactor," in situ hydrogenation, and conversion utilizing hydrogen and/or a hydrogen donor solvent, recovery of the converted and produced crude, separation at the surface into various fractions, and utilization of the heavy residual fraction to produce hydrogen for re-injection. Heating of the injected fluids is accomplished on the surface which, as discussed above, is an inefficient process. PA1 Ware, U.S. Pat. No. 4,597,441 describes in situ "hydrogenation" (defined as the addition of hydrogen to the oil without cracking) and "hydrogenolysis" (defined as hydrogenation with simultaneous cracking). Ware teaches the use of a downhole combustor. Reference is made to previous patents relating to a gas generator of the type disclosed in U.S. Pat. Nos. 3,982,591; 3,982,592; or 4,199,024. Ware further teaches and claims injection from the combustor of superheated steam and hydrogen to cause hydrogenation of petroleum in the formation. Ware also stipulates that after injecting superheated steam and hydrogen, sufficient pressure is maintained "to retain the hydrogen in the heated formation zone in contact with the petroleum therein for `soaking` purposes for a period of time." In some embodiments Ware includes combustion of petroleum products in the formation--a major disadvantage, as discussed earlier--to drive fluids from the injection to the production wells. PA1 Richardson, U.S. Pat. No. 4,160,479 teaches the use of a produced residuum fraction as a feed to a gasifier for the production of energy; i.e., power, steam, etc. Hot gases produced are available for injection at a pressure of 150 atmospheres and temperatures between 800 and 1,000.degree. C. Hydrogen and oxygen are produced by electrolytic hydrolysis of water. PA1 Sweany, U.S. Pat. No. 4,284,139 teaches the use of a produced residuum fraction (pitch) which is subjected to partial oxidation to produce hydrogen and steam. Sweany utilizes surface upgrading accomplished in the presence of a hydrogen donor on the surface. PA1 Hyne, U.S. Pat. No. 4,487,264 injects steam at a temperature of 260.degree. C. or less to promote the water-gas-shift reaction to form in situ carbon dioxide and hydrogen. Hyne claims that the long-term exposure of heavy oil to polymerization, degradation, etc. is reduced due to the formation hydrocarbon's exposure to less elevated temperatures. PA1 Schora, U.S. Pat. No. 4,141,417 injects hydrogen and carbon dioxide at a temperature of less than 300.degree. F. and claims to reduce the hydrocarbon formation viscosity and accomplish desulfurization. Viscosity reduction is assumed primarily through the well-known mechanism involving solution of carbon dioxide in the hydrocarbon. PA1 a. inserting downhole combustion units within injection boreholes, which communicate with production boreholes by means of horizontal fractures, at or near the level of the subsurface formation containing a heavy hydrocarbon; PA1 b. for a preheat period, flowing from the surface through said injection boreholes stoichiometric proportions of a reducing-gas mixture and an oxidizing fluid to said downhole combustion units and igniting same in said downhole combustion units to produce hot combustion gases, including superheated steam, while flowing partially saturated steam from the surface through said injection boreholes to said downhole combustion units to control the temperature of said heated gases and to produce additional superheated steam; PA1 c. injecting said superheated steam into the subsurface formation to heat a region of the subsurface formation to a preferred temperature; PA1 d. for a conversion period, increasing the ratio of reducing gas to oxidant in the mixture fed to the downhole combustion units, or injecting reducing gas in the fluid stream controlling the temperature of the combustion units, to provide an excess of reducing gas in the hot gases exiting the combustion units; PA1 e. continuously injecting the heated excess reducing gas and superheated steam into the subsurface formation to provide preferred conditions and reactants to sustain in situ hydrovisbreaking and thereby upgrade the heavy hydrocarbon; PA1 f. collecting continuously at the surface, from said production boreholes, production fluids comprised of converted liquid hydrocarbons, unconverted virgin heavy hydrocarbons, residual reducing gases, hydrocarbon gases, solids, water, hydrogen sulfide, and other components for further processing; PA1 g. treating at the surface the said production fluids to recover thermal energy and to separate produced solids, gases, and produced liquid hydrocarbons; PA1 h. fractionating the said produced liquid hydrocarbons to provide an upgraded liquid hydrocarbon product and a heavy residuum fraction; PA1 i. carrying out partial oxidation of said residuum fraction and gas-treating operations to produce a clean reducing gas mixture and a fuel gas stream; PA1 j. carrying out treating operations on the separated gases and residual reducing-gas mixture to remove water, hydrogen sulfide, and other undesirable components and to separate hydrocarbon gases and residual reducing gas mixture; PA1 k. combining said reducing gas mixtures of steps i and j to form the reducing gas mixture of step b; PA1 l. generation of steam using as fuel the combined hydrocarbon gases of step j and fuel gas of step f; PA1 m. repeating steps d through l.
The present invention concerns a process conducted at the surface which treats the raw production recovered from the application of in situ hydrovisbreaking to a heavy-hydrocarbon deposit. The process of this invention produces a synthetic crude oil (or "syncrude") with a nominal boiling range of butane (C.sub.4) to 975.degree. F., making it a suitable feedstock for transportation fuels and gas oil. The process also produces a heavy residuum stream (a nominal 975.degree. F.+ fraction) which is processed further to produce the energy and reactants required for the application of in situ hydrovisbreaking.
Following is a review of the prior art as related to the operations relevant to this invention. The patents referenced teach or suggest the use of a downhole apparatus for in situ operations, procedures for effecting in situ conversion of heavy crudes and bitumens, and methods for recovering and processing the produced hydrocarbons.
Some of the best prior art disclosing the use of downhole devices for secondary recovery is found in U.S. Pat. Nos. 4,159,743; 5,163,511; 4,865,130; 4,691,771; 4,199,024; 4,597,441; 3,982,591; 3,982,592; 4,024,912; 4,053,015; 4,050,515; 4,077,469; and 4,078,613. Other expired patents which also disclose downhole generators for producing hot gases or steam are U.S. Pat. Nos. 2,506,853; 2,584,606; 3,372,754; 3,456,721; 3,254,721; 2,887,160; 2,734,578; and 3,595,316.
The concept of separating produced secondary crude oil into hydrogen, lighter oils, etc. and the use of hydrogen for in situ combustion and downhole steaming operations to recover hydrocarbons are found in U.S. Pat. Nos. 3,707,189; 3,908,762; 3,986,556; 3,990,513; 4,448,251; 4,476,927; 3,051,235; 3,084,919; 3,208,514; 3,327,782; 2,857,002; 4,444,257; 4,597,441; 4,241,790; 4,127,171; 3,102,588; 4,324,291; 4,099,568; 4,501,445; 3,598,182; 4,148,358; 4,186,800; 4,233,166; 4,284,139; 4,160,479; and 3,228,467. Additionally, in situ hydrogenation with hydrogen or a reducing gas is taught in U.S. Pat. Nos. 5,145,003; 5,105,887; 5,054,551; 4,487,264; 4,284;139; 4,183,405; 4,160,479; 4,141,417; 3,617,471; and 3,228,467.
U.S. Pat. No. 3,598,182 to Justheim; U.S. Pat. No. 3,327,782 to Hujsak; U.S. Pat. No. 4,448,251 to Stine; U.S. Pat. No. 4,501,445 to Gregoli; and U.S. Pat. No. 4,597,441 to Ware all teach variations of in situ hydrogenation which more closely resemble the current invention:
None of these patents disclose an integrated process in which heavy hydrocarbons are converted in situ to lighter hydrocarbons by injecting steam and hot reducing gases with the produced hydrocarbons separated at the surface into various fractions and the residuum fraction diverted for the production of reducing gas and steam while the lighter hydrocarbon fractions are marketed as a source for transportation fuels and gas oil.
Another group of U.S. patents--including U.S. Pat. Nos. 5,145,003 and 5,054,551 to Duerksen; U.S. Pat. No. 4,160,479 to Richardson; U.S. Pat. No. 4,284,139 to Sweany; U.S. Pat. No. 4,487,264 to Hyne; and U.S. Pat. No. 4,141,417 to Schora--all teach variations of hydrogenation with heating of the injected fluids (hydrogen, reducing gas, steam, etc.) accomplished at the surface. Further:
In addition to not using a downhole combustion unit for injection of hot reducing gases, none of these patents includes the processing of a syncrude product with the properties claimed in this invention. Most importantly, none of the patents referenced herein includes the unique and novel integration of in situ hydrovisbreaking with the operations comprising in this invention.
All of the U.S. patents mentioned are fully incorporated herein by reference thereto as if fully repeated verbatim immediately hereafter.
In light of the current state of the technology, what is needed--and what has been discovered by us--is a unique process for producing valuable petroleum products, such as syncrude boiling in the transportation-fuel range (C.sub.4 to 650.degree. F.) and gas-oil range (650 to 975.degree. F.) from the raw production of heavy crudes and bitumens with the energy and reactants used in the recovery operation produced from the less desirable components of the raw production. The process disclosed in this invention minimizes the amount of surface processing required to produce marketable petroleum products while permitting the production and utilization of hydrocarbon resources which are otherwise not economically recoverable.
Objectives of the Invention
The primary objective of this invention is to provide a process for producing a synthetic crude oil that is a suitable feedstock for transportation fuels and gas oil from the raw production of heavy crude oils and natural bitumens recovered by the application in situ hydrovisbreaking.
Another objective of this invention is to enhance the quality of the partially upgraded hydrocarbons produced from the formation by above-ground removal of the heavy residuum fraction and the carbon residue contained in the produced hydrocarbons. This results in the production of a more valuable syncrude product with reduced levels of sulfur, nitrogen, and metals.
The in situ hydrovisbreaking operation utilizes downhole combustion units. A further objective of this invention is to utilize the separated residuum fraction as a feedstock for a partial oxidation operation to provide clean hydrogen for combustion in the downhole combustion units and injection into the hydrocarbon-bearing formation as well as fuel gas for use in steam and electric power generation.